Chemical reactions

A chemical reaction can be said to occur whenever there is any change in a chemical substance. Reactions are at the heart of chemistry. If you have studied any chemistry at all you will have been asked to learn a number of chemical reactions. Certainly we cannot proceed very far in environmental study without dealing with chemical reactions, because they are occurring around us continually. First here, we introduce the ways in which chemical reactions are conventionally expressed. Consider the following expression:

CaSO₄ + C → CaS + CO

This simply tells us that when calcium sulphate is combined with carbon, calcium sulphide and carbon monoxide are produced, the arrow indicating the direction of the reaction. In other words, symbols for elements and compounds are used rather than the written name, which would be cumbersome. But the two sides do not balance: there are four oxygen atoms on the left-hand side of the arrow and only one on the right. (The subscript 4 indicates the number of atoms of the preceding element that is present in the molecule or compound. If no subscripted number is shown, this implies that one atom is present. Thus, SO₄ contains one sulphur atom and four oxygen atoms.) To provide an equation, in which the total numbers of atoms of each element are equal on both sides, numbers are placed before the appropriate compound symbols, thus:

CaSO₄ + 4C → CaS + 4CO